Find the first derivative of f when f(x) = cos2x−3sin2x.

1. f′(x) = 10sin2x

2. f′(x) = −10cos2x

3. f′(x) = 5sin2x

4. f′(x) = −10sin2x

5. f′(x) = −5cos2x

6. f′(x) = −5sin2x

thanks!